96 HUMAN PHYSIOLOGY 



represent the end of the renal artery and the begin- 

 ning of the renal vein. The blood which, so to 

 speak, circulates in and out of the kidney, finds its 

 turning point in the little bodies (Malpighian bodies) 

 just noted. These structures are lined by cells which 

 represent the active agents in the kidney's work. 

 The little artery entering the Malpighian body is 

 wider in diameter than the vein which is given off, 

 hence the blood is forced at a certain pressure into 

 these bodies, and in this mechanical fashion the waste 

 matters are excreted, especially water, which forms 

 the great bulk of the kidney secretion or urine. 

 Each of the Malpighian bodies is in fact a kind of 

 filter which filters out from the blood the waste 

 matters of the kidney secretion. From each of these 

 bodies a tube passes out in a varied and somewhat 

 complicated arrangement to the centre of the kidney, 

 the multitude of tubes ending in the hollow of the 

 kidney on certain little projections or papilla. It 

 would seem that the fluid part of the urine or water 

 is really forced from the blood by a process of 

 filtration within the little bodies described, whilst 

 the other matters removed from the blood are in all 

 probability separated by the cells of the Malpighian 

 bodies themselves, and by those which line the tubes 

 leading from the bodies in question and which 

 conduct the urine to the outlet of the kidney. From 

 each kidney a special tube, the ureter, leads to the 

 bladder in which the urine is stored preparatory to 

 its expulsion. 



THE KIDNEY SECRETION. A healthy adult will 

 pass from the kidneys in twenty-four hours between 

 fifty and sixty fluid ounces of urine, although the 

 quantity varies materially according to the circum- 

 stances of life, and even according to the outer 



